The Hemoglobin A1C test, commonly known as A1C, is a standard measure for diagnosing and monitoring diabetes. Gestational Diabetes Mellitus (GDM) is a temporary condition where blood glucose levels rise during pregnancy, distinct from pre-existing Type 1 or Type 2 Diabetes. While A1C provides a long-term picture of average blood sugar control, the unique physiological changes of pregnancy interfere with its accuracy. Therefore, A1C is not the primary diagnostic tool for GDM. The connection between A1C and GDM is complex, shifting from a limited role during pregnancy to a highly significant one after delivery.
Understanding A1C and Gestational Diabetes
Gestational Diabetes is defined as glucose intolerance that begins or is first recognized during pregnancy. The condition arises because placental hormones, such as human placental lactogen and cortisol, create a state of insulin resistance. If the pancreas cannot produce enough insulin to manage blood glucose, GDM develops. For most affected women, this condition resolves immediately after the baby is born.
The A1C test measures a person’s average blood glucose levels over the preceding two to three months. It works by measuring the percentage of hemoglobin—the oxygen-carrying protein in red blood cells—that has glucose attached (glycation). Since red blood cells typically live for about 120 days, the A1C result reflects the average glucose exposure over that time frame. A higher A1C percentage indicates higher average blood sugar levels.
Diagnostic Role: Why A1C Isn’t Used for GD Screening
The standard method for diagnosing GDM in the second trimester is the Oral Glucose Tolerance Test (OGTT), typically administered between 24 and 28 weeks of gestation. The OGTT measures the body’s immediate response to sugar, which better indicates the acute insulin resistance caused by placental hormones. A1C is generally not used for mid-pregnancy screening because pregnancy alters the body’s blood components, making the result unreliable.
One significant factor is the accelerated turnover of red blood cells (RBCs) during pregnancy. The typical 120-day lifespan of RBCs is shortened, sometimes to around 90 days. Since the A1C relies on the full lifespan of the red blood cell to accurately reflect a three-month average, this shortened lifespan means the result reflects a much shorter period. This physiological change can lead to a falsely low A1C reading, even if blood sugar levels are rising.
Hemodilution is another interfering factor, caused by the increase in total blood volume during a healthy pregnancy. This increased volume effectively dilutes the concentration of hemoglobin in the blood. Hemodilution contributes to the artificial lowering of the A1C percentage. Due to these combined effects of shortened RBC life and blood dilution, the A1C can significantly underestimate the true degree of glucose intolerance in mid-to-late pregnancy.
Monitoring and Risk Assessment During Pregnancy
While A1C is unreliable for GDM diagnosis in the second trimester, it serves an important purpose earlier in pregnancy. The test is often used during the first prenatal visit to screen for undiagnosed, pre-existing Type 2 Diabetes or prediabetes. A first-trimester A1C reading of 6.5% or higher indicates overt diabetes. Readings between 5.7% and 6.4% suggest prediabetes. Identifying this condition early is necessary because it requires different management than GDM alone.
An A1C result in the prediabetes range early in pregnancy identifies women at a significantly higher risk of subsequently developing GDM. For women already diagnosed with GDM, A1C may be used as a secondary tool in limited circumstances to assess long-term glucose control. If a patient’s self-monitored blood glucose records are inconsistent or unavailable, the A1C provides a supplemental measure of average control. However, it is not the primary way to track daily glucose management, which is best done through frequent self-monitoring or continuous glucose monitors.
Post-Pregnancy A1C: Assessing Future Risk
The most significant connection between GDM and the A1C test occurs after delivery. A diagnosis of GDM indicates a major predisposition for developing Type 2 Diabetes (T2D) later in life. Women who have had GDM face a seven-fold increased risk of developing T2D compared to those who did not. Post-pregnancy screening is a necessary part of long-term health management due to this increased risk.
The A1C test becomes a reliable, standard tool again once the pregnancy-related physiological changes have resolved. Healthcare providers recommend a screening test—A1C, a fasting plasma glucose test, or an OGTT—at a follow-up visit, typically between 6 and 12 weeks postpartum. This test determines if the glucose intolerance has fully resolved or if the patient has transitioned to T2D or prediabetes.
If the A1C test is used in the immediate postpartum period (generally within the first six months), it may still be less accurate. For this reason, if an A1C is used during this early window, a repeat test is often indicated to confirm the result. Women with a normal result should still be rescreened at least every three years for a minimum of ten years after the pregnancy, using the A1C test as a standard marker for ongoing T2D risk.